The present invention relates to self-calibrating temperature probes that are used in thermal processing systems.
In rapid thermal processing (RTP), a substrate is heated quickly and uniformly to a high temperature, such as 400.degree. C. to 1200.degree. C., to perform a fabrication step such as annealing, cleaning, chemical vapor deposition, oxidation, or nitration. For example, a thermal processing system, such as the RTP tool available from Applied Materials under the trade name "Centura", may be used to perform metal annealing at temperatures of 400.degree. C. to 500.degree. C., titanium silicide formation at temperatures around 650.degree. C., or oxidation or implant annealing at temperatures around 1000.degree. C.
The temperature of the substrate must be precisely controlled during these thermal processing steps to obtain high yields and process reliability, particularly given the submicron dimension of current devices. For example, to fabricate a dielectric layer 60-80 .ANG. thick with a uniformity of .+-.2 .ANG. (a typical requirement in current device structures), the temperature in successive processing runs cannot vary by more than a few .degree.C. from the target temperature. To achieve this level of temperature control, the temperature of the substrate is measured in real time and in situ.
Optical pyrometry is a technology that is used to measure substrate temperatures in RTP systems. In optical pyrometry, a light pipe of a temperature probe samples the radiation emitted from the substrate, and a pyrometer computes the temperature of the substrate based on the intensity of the sampled radiation, the spectral emissivity of the substrate, and the ideal blackbody radiation-temperature relationship.
Assuming the temperature probe is initially calibrated to produce a correct temperature reading, repeated use may cause the temperature sensed by the probe to change over time. For example, the light pipe may become dirty or chipped, the electronic components in the pyrometer may "drift", or the connections along the optical path from the light pipe to the pyrometer may loosen. Thus it will be necessary to recalibrate the probe or at least detect the change that has occurred so that corrective action can be taken.
Even if the temperature probe remains calibrated, the processing chamber may change and introduce an error into the measured temperature of the substrate. One common component of the processing chamber is a reflector plate positioned beneath the substrate to form a reflecting cavity therebetween. This reflector plate causes radiation from the substrate to be reflected back to the substrate. It can be shown that if the reflector plate were an ideal reflector, all of the radiation emitted by the substrate would be reflected back to the substrate, and the reflecting cavity would act like an ideal black body. That is, the reflector plate affects the effective emissivity of the substrate.
As a result of processing operations, the reflector plate may become dirty or corroded, and thus less reflective. If the reflectivity of the reflector plate changes, the effective emissivity of the substrate also changes. The variation in the effective emissivity of the substrate changes the intensity of the radiation sampled by the temperature probe, and creates an error in the measured temperature. One method of detecting changes in the reflector plate is visual inspection (either by the human eye or under microscope). To provide access for visual inspection of the reflector plate, the processing chamber is opened.
Another obstruction to the accurate measurement of substrate temperatures is the differences in roughness between substrates. The roughness of the substrate can affect the intensity of the light sampled by the temperature probe. Because in situ determination of the roughness of a substrate has, as yet, been impractical, the pyrometer computes the substrate temperature as if each substrate had the same roughness.